In 2003, Chicago launched “Double-Dose Algebra,” requiring students with pretest scores below the national median to take two periods of math–algebra and supplemental coursework. In many schools, assignment to Double Dose changed the peer composition of the algebra classroom. Using school-specific instrumental variables within a regression-discontinuity design (RDD), we find that attending a lower skill classroom reduced math achievement for median-skill students. As a result, the Double-Dose policy had little or no effect for median-skill students in schools that exposed them to low-skill classrooms. However, the effects of Double Dose were substantially positive in schools that did not do so. We consider policy implications and interpretations of the results from RDDs.